The invention relates to friction clutches in general, and more particularly to improvements in friction clutches which employ or which cooperate with so-called clutch plates or clutch discs having friction linings which can be clamped between an axially movable rotary pressure plate and a driven counterpressure plate (such as a flywheel) so that the thus engaged clutch can transmit torque between the clutch disc and a prime mover for the counterpressure plate. Still more particularly, the invention relates to improvements in friction clutches which are equipped or which cooperate with adjusting units designed to compensate for wear at least upon the friction linings of the clutch disc. Such adjusting units are disclosed, for example, in published German patent applications Nos. 42 39 291, 43 06 505, 42 43 289, 43 42 390 and 43 22 677 to which reference may be had if necessary. Additional friction clutches with wear compensating units are disclosed in commonly owned U.S. Pat. No. 5,450,934 the disclosure of which is incorporated herein by reference.
When a friction clutch of the above outlined character is engaged, the pressure plate is caused to bear against the adjacent friction linings of the clutch disc with a considerable force. On the other hand, it is desirable to design the friction clutch in such a way that a relatively small force is required to disengage the pressure plate from the friction linings and that this relatively small disengaging force remains at least substantially unchanged during the entire useful life of the clutch (such useful life is determined, at least to a large degree, by the extent of wear upon the friction linings of the clutch disc). As a rule, the magnitude of such disengaging force should remain within a relatively narrow range of tolerances during the life span of the friction clutch. Pronounced fluctuations of the magnitude of disengaging force are undesirable for a number of reasons; for example, the operator of a motor vehicle which is equipped with a foot-operated friction clutch should encounter a preferably unchanging resistance to depression of the clutch pedal irrespective of the extent of wear upon the friction linings of the clutch disc. This enables the operator of the vehicle to actuate the clutch pedal with a required degree of reliability and predictability.
As a rule, the means for biasing the pressure plate against the friction linings of the clutch disc comprises or constitutes a diaphragm spring. In order to ensure that the diaphragm spring will bias the pressure plate with a pronounced force when the friction clutch is fully engaged but that the disengagement of the clutch will involve the application of a relatively small and at least substantially constant force, the diaphragm spring must be designed and installed in such a way that its characteristic curve exhibits a pronounced drop in that region which is relevant during disengagement of the clutch. Furthermore, and in addition to being required to ensure a disengagement of the clutch in response to the exertion of a relatively small force to the clutch pedal, the diaphragm spring should be designed to establish the possibility of at least some slight additional axial movement of the pressure plate when the disengaging operation is completed in order to account for eventual tolerances in the making and/or in the assembly of the friction clutch. It has been found that presently known friction clutches which employ diaphragm springs and are equipped with means for compensating for wear at least upon the friction linings of the clutch disc do not satisfy all of the above-enumerated requirements. One of the main reasons for such failure of heretofore known friction clutches to satisfy all of the above outlined requirements regarding the magnitude of the disengaging force is that, though the characteristic curve of the diaphragm spring exhibits a pronounced downward slope during actual disengagement of the clutch, the curve exhibits an immediately following pronounced upward slope, i.e., the clutch disengaging force is relatively low during an initial stage of disengagement but becomes rather pronounced or even very pronounced during the next-following stage. This can be a cause of discomfort to the occupant or occupants of a motor vehicle having a power train which embodies a friction clutch of the above outlined character.